using System.Threading;
using System;
using System.Linq;
using System.Net;
using System.Net.Sockets;
using System.Security;
using System.Text;
using System.Net.Security;
using System.Threading.Tasks;
using System.Collections.Generic;
#if !NETSTANDARD1_3
using System.Net.Mail;
#else
using Exceptions;
#endif

namespace Unosquare.Swan.Networking {
  /// <summary>
  /// Represents a basic SMTP client that is capable of submitting messages to an SMTP server.
  /// </summary>
  /// <example>
  /// The following code explains how to send a simple e-mail.
  /// <code>
  /// using System.Net.Mail;
  ///  
  /// class Example
  /// {
  ///     static void Main()
  ///     {
  ///         // create a new smtp client using google's smtp server
  ///         var client = new SmtpClient("smtp.gmail.com", 587);
  ///         
  ///         // send an email 
  ///         client.SendMailAsync(
  ///         new MailMessage("sender@test.com", "recipient@test.cm", "Subject", "Body"));
  ///     }
  /// }
  /// </code>
  /// 
  /// The following code demonstrates how to sent an e-mail using a SmtpSessionState:
  /// <code>
  /// class Example
  /// {
  ///     static void Main()
  ///     {
  ///         // create a new smtp client using google's smtp server
  ///         var client = new SmtpClient("smtp.gmail.com", 587);
  ///         
  ///         // create a new session state with a sender address 
  ///         var session = new SmtpSessionState { SenderAddress = "sender@test.com" };
  ///         
  ///         // add a recipient
  ///         session.Recipients.Add("recipient@test.cm");
  ///         
  ///         // send
  ///         client.SendMailAsync(session);
  ///     }
  /// }
  /// </code>
  /// 
  /// The following code shows how to send an e-mail with an attachment:
  /// <code>
  /// using System.Net.Mail;
  ///  
  /// class Example
  /// {
  ///     static void Main()
  ///     {
  ///         // create a new smtp client using google's smtp server
  ///         var client = new SmtpClient("smtp.gmail.com", 587);
  ///         
  ///         // create a new session state with a sender address 
  ///         var session = new SmtpSessionState { SenderAddress = "sender@test.com" };
  ///         
  ///         // add a recipient
  ///         session.Recipients.Add("recipient@test.cm");
  ///         
  ///         // load a file as an attachment
  ///         var attachment = new MimePart("image", "gif")
  ///         {
  ///             Content = new 
  ///                 MimeContent(File.OpenRead("meme.gif"), ContentEncoding.Default),
  ///             ContentDisposition = 
  ///                 new ContentDisposition(ContentDisposition.Attachment),
  ///             ContentTransferEncoding = ContentEncoding.Base64,
  ///             FileName = Path.GetFileName("meme.gif")
  ///         };
  ///         
  ///         // send
  ///         client.SendMailAsync(session);
  ///     }
  /// }
  /// </code>
  /// </example>
  public class SmtpClient {
    /// <summary>
    /// Initializes a new instance of the <see cref="SmtpClient" /> class.
    /// </summary>
    /// <param name="host">The host.</param>
    /// <param name="port">The port.</param>
    /// <exception cref="ArgumentNullException">host.</exception>
    public SmtpClient(String host, Int32 port) {
      this.Host = host ?? throw new ArgumentNullException(nameof(host));
      this.Port = port;
      this.ClientHostname = Network.HostName;
    }

    /// <summary>
    /// Gets or sets the credentials. No credentials will be used if set to null.
    /// </summary>
    /// <value>
    /// The credentials.
    /// </value>
    public NetworkCredential Credentials {
      get; set;
    }

    /// <summary>
    /// Gets the host.
    /// </summary>
    /// <value>
    /// The host.
    /// </value>
    public String Host {
      get;
    }

    /// <summary>
    /// Gets the port.
    /// </summary>
    /// <value>
    /// The port.
    /// </value>
    public Int32 Port {
      get;
    }

    /// <summary>
    /// Gets or sets a value indicating whether the SSL is enabled.
    /// If set to false, communication between client and server will not be secured.
    /// </summary>
    /// <value>
    ///   <c>true</c> if [enable SSL]; otherwise, <c>false</c>.
    /// </value>
    public Boolean EnableSsl {
      get; set;
    }

    /// <summary>
    /// Gets or sets the name of the client that gets announced to the server.
    /// </summary>
    /// <value>
    /// The client hostname.
    /// </value>
    public String ClientHostname {
      get; set;
    }

#if !NETSTANDARD1_3
    /// <summary>
    /// Sends an email message asynchronously.
    /// </summary>
    /// <param name="message">The message.</param>
    /// <param name="sessionId">The session identifier.</param>
    /// <param name="ct">The cancellation token.</param>
    /// <param name="callback">The callback.</param>
    /// <returns>
    /// A task that represents the asynchronous of send email operation.
    /// </returns>
    /// <exception cref="ArgumentNullException">message.</exception>
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Codequalität", "IDE0067:Objekte verwerfen, bevor Bereich verloren geht", Justification = "<Ausstehend>")]
    public Task SendMailAsync(
        MailMessage message,
        String sessionId = null,
        CancellationToken ct = default,
        RemoteCertificateValidationCallback callback = null) {
      if(message == null) {
        throw new ArgumentNullException(nameof(message));
      }

      SmtpSessionState state = new SmtpSessionState {
        AuthMode = this.Credentials == null ? String.Empty : SmtpDefinitions.SmtpAuthMethods.Login,
        ClientHostname = ClientHostname,
        IsChannelSecure = EnableSsl,
        SenderAddress = message.From.Address,
      };

      if(this.Credentials != null) {
        state.Username = this.Credentials.UserName;
        state.Password = this.Credentials.Password;
      }

      foreach(MailAddress recipient in message.To) {
        state.Recipients.Add(recipient.Address);
      }

      state.DataBuffer.AddRange(message.ToMimeMessage().ToArray());

      return this.SendMailAsync(state, sessionId, ct, callback);
    }
#endif

    /// <summary>
    /// Sends an email message using a session state object.
    /// Credentials, Enable SSL and Client Hostname are NOT taken from the state object but
    /// rather from the properties of this class.
    /// </summary>
    /// <param name="sessionState">The state.</param>
    /// <param name="sessionId">The session identifier.</param>
    /// <param name="ct">The cancellation token.</param>
    /// <param name="callback">The callback.</param>
    /// <returns>
    /// A task that represents the asynchronous of send email operation.
    /// </returns>
    /// <exception cref="ArgumentNullException">sessionState.</exception>
    public Task SendMailAsync(
        SmtpSessionState sessionState,
        String sessionId = null,
        CancellationToken ct = default,
        RemoteCertificateValidationCallback callback = null) {
      if(sessionState == null) {
        throw new ArgumentNullException(nameof(sessionState));
      }

      return this.SendMailAsync(new[] { sessionState }, sessionId, ct, callback);
    }

    /// <summary>
    /// Sends an array of email messages using a session state object.
    /// Credentials, Enable SSL and Client Hostname are NOT taken from the state object but
    /// rather from the properties of this class.
    /// </summary>
    /// <param name="sessionStates">The session states.</param>
    /// <param name="sessionId">The session identifier.</param>
    /// <param name="ct">The cancellation token.</param>
    /// <param name="callback">The callback.</param>
    /// <returns>
    /// A task that represents the asynchronous of send email operation.
    /// </returns>
    /// <exception cref="ArgumentNullException">sessionStates.</exception>
    /// <exception cref="SecurityException">Could not upgrade the channel to SSL.</exception>
    /// <exception cref="SmtpException">Defines an SMTP Exceptions class.</exception>
    public async Task SendMailAsync(
        IEnumerable<SmtpSessionState> sessionStates,
        String sessionId = null,
        CancellationToken ct = default,
        RemoteCertificateValidationCallback callback = null) {
      if(sessionStates == null) {
        throw new ArgumentNullException(nameof(sessionStates));
      }

      using(TcpClient tcpClient = new TcpClient()) {
        await tcpClient.ConnectAsync(this.Host, this.Port).ConfigureAwait(false);

        using(Connection connection = new Connection(tcpClient, Encoding.UTF8, "\r\n", true, 1000)) {
          SmtpSender sender = new SmtpSender(sessionId);

          try {
            // Read the greeting message
            sender.ReplyText = await connection.ReadLineAsync(ct).ConfigureAwait(false);

            // EHLO 1
            await this.SendEhlo(ct, sender, connection).ConfigureAwait(false);

            // STARTTLS
            if(this.EnableSsl) {
              sender.RequestText = $"{SmtpCommandNames.STARTTLS}";

              await connection.WriteLineAsync(sender.RequestText, ct).ConfigureAwait(false);
              sender.ReplyText = await connection.ReadLineAsync(ct).ConfigureAwait(false);
              sender.ValidateReply();

              if(await connection.UpgradeToSecureAsClientAsync(callback: callback).ConfigureAwait(false) == false) {
                throw new SecurityException("Could not upgrade the channel to SSL.");
              }
            }

            // EHLO 2
            await this.SendEhlo(ct, sender, connection).ConfigureAwait(false);

            // AUTH
            if(this.Credentials != null) {
              ConnectionAuth auth = new ConnectionAuth(connection, sender, this.Credentials);
              await auth.AuthenticateAsync(ct).ConfigureAwait(false);
            }

            foreach(SmtpSessionState sessionState in sessionStates) {
              {
                // MAIL FROM
                sender.RequestText = $"{SmtpCommandNames.MAIL} FROM:<{sessionState.SenderAddress}>";

                await connection.WriteLineAsync(sender.RequestText, ct).ConfigureAwait(false);
                sender.ReplyText = await connection.ReadLineAsync(ct).ConfigureAwait(false);
                sender.ValidateReply();
              }

              // RCPT TO
              foreach(String recipient in sessionState.Recipients) {
                sender.RequestText = $"{SmtpCommandNames.RCPT} TO:<{recipient}>";

                await connection.WriteLineAsync(sender.RequestText, ct).ConfigureAwait(false);
                sender.ReplyText = await connection.ReadLineAsync(ct).ConfigureAwait(false);
                sender.ValidateReply();
              }

              {
                // DATA
                sender.RequestText = $"{SmtpCommandNames.DATA}";

                await connection.WriteLineAsync(sender.RequestText, ct).ConfigureAwait(false);
                sender.ReplyText = await connection.ReadLineAsync(ct).ConfigureAwait(false);
                sender.ValidateReply();
              }

              {
                // CONTENT
                String dataTerminator = sessionState.DataBuffer
                    .Skip(sessionState.DataBuffer.Count - 5)
                    .ToText();

                sender.RequestText = $"Buffer ({sessionState.DataBuffer.Count} bytes)";

                await connection.WriteDataAsync(sessionState.DataBuffer.ToArray(), true, ct).ConfigureAwait(false);
                if(dataTerminator.EndsWith(SmtpDefinitions.SmtpDataCommandTerminator) == false) {
                  await connection.WriteTextAsync(SmtpDefinitions.SmtpDataCommandTerminator, ct).ConfigureAwait(false);
                }

                sender.ReplyText = await connection.ReadLineAsync(ct).ConfigureAwait(false);
                sender.ValidateReply();
              }
            }

            {
              // QUIT
              sender.RequestText = $"{SmtpCommandNames.QUIT}";

              await connection.WriteLineAsync(sender.RequestText, ct).ConfigureAwait(false);
              sender.ReplyText = await connection.ReadLineAsync(ct).ConfigureAwait(false);
              sender.ValidateReply();
            }
          } catch(Exception ex) {
            String errorMessage =
                $"Could not send email. {ex.Message}\r\n    Last Request: {sender.RequestText}\r\n    Last Reply: {sender.ReplyText}";
            errorMessage.Error(typeof(SmtpClient).FullName, sessionId);

            throw new SmtpException(errorMessage);
          }
        }
      }
    }

    private async Task SendEhlo(CancellationToken ct, SmtpSender sender, Connection connection) {
      sender.RequestText = $"{SmtpCommandNames.EHLO} {this.ClientHostname}";

      await connection.WriteLineAsync(sender.RequestText, ct).ConfigureAwait(false);

      do {
        sender.ReplyText = await connection.ReadLineAsync(ct).ConfigureAwait(false);
      } while(!sender.IsReplyOk);

      sender.ValidateReply();
    }

    private class ConnectionAuth {
      private readonly SmtpSender _sender;
      private readonly Connection _connection;
      private readonly NetworkCredential _credentials;

      public ConnectionAuth(Connection connection, SmtpSender sender, NetworkCredential credentials) {
        this._connection = connection;
        this._sender = sender;
        this._credentials = credentials;
      }

      public async Task AuthenticateAsync(CancellationToken ct) {
        this._sender.RequestText =
            $"{SmtpCommandNames.AUTH} {SmtpDefinitions.SmtpAuthMethods.Login} {Convert.ToBase64String(Encoding.UTF8.GetBytes(this._credentials.UserName))}";

        await this._connection.WriteLineAsync(this._sender.RequestText, ct).ConfigureAwait(false);
        this._sender.ReplyText = await this._connection.ReadLineAsync(ct).ConfigureAwait(false);
        this._sender.ValidateReply();
        this._sender.RequestText = Convert.ToBase64String(Encoding.UTF8.GetBytes(this._credentials.Password));

        await this._connection.WriteLineAsync(this._sender.RequestText, ct).ConfigureAwait(false);
        this._sender.ReplyText = await this._connection.ReadLineAsync(ct).ConfigureAwait(false);
        this._sender.ValidateReply();
      }
    }
  }
}